Much effort has recently been expended on producing glass by sol-gel processes. For a recent partial review of the field, see, for instance, D. R. Uhlmann et al, in Better Ceramics Through Chemistry, Materials Research Society Symposia Proceedings, Vol. 32, C. J. Brinker et al, editor, (1984), pp. 59-70.
Sol-gel methods for producing glass precursor material can be divided into techniques that form a gel using pre-existing colloidal particles (e.g., fumed silica), and into techniques that form a gel by hydrolization and polymerization of appropriate chemical compounds. This application is concerned with techniques in the latter category, and with a combination of the two techniques. The techniques of concern herein will be collectively referred to as "polymerization" techniques.
Polymerization techniques may result in gel formation even though a classical sol may not have been formed in the process. It is customary, however, to refer to these processes as sol-gel processes. Typically, gels formed by polymerization techniques are among the monolithic gels. Such a gel may be pictured as a continuous molecular network, having a sponge-like structure, with liquid occluded in the interstices of the "sponge".
Prominent among the known polymerization techniques is the alkoxide method, which is reviewed, for instance, by S. Sakka, Treatise on Material Science and Technology, Vol. 22, Academic Press (1982), pp. 129-167. See also L. C. Klein et al, Soluble Silicates, ACS Symposium Series, Vol. 194, American Chemical Society (1982), pp. 293-304.
Briefly, the alkoxide method comprises mixing an alkoxide, typically a silicon alkoxide such as tetraethylorthosilicate (TEOS), an alcohol such as ethanol, and water. An electrolyte such as HCl or NH.sub.4 OH may also be added. Mixing of the components results in formation of a clear "sol", with hydrolysis occurring rapidly, followed by polymerization, a result of whichis increasing viscosity of the sol, with the viscous sol becoming an elastic gel in which all of the liquid phase is occluded by a substantially continuous network of the solid phase. Typically, the gel is dried, resulting in formation of a porous, relatively rigid solid, which can be transformed into a glass by sintering at a temperature substantially below the conventional softening temperature of the glass. If desired, adsorbed water and hydroxyl ions can be removed, prior to or during sintering, by means of a known chlorine treatment. The alkoxide process can be used to produce doped high-silica glass and some mixed oxide glasses, provided the dopants or other necessary constituents are available in alkoxide form or in the form of water or alcohol-soluble salts.
Various techniques are known for producing a glass body from material produced by the alkoxide method. Among these techniques are the double dispersion method of U.S. Pat. No. 4,419,115, and the particle fusion technique of U.S. Pat. No. 3,954,431, incorporated herein by reference, both co-assigned with this. See also S. Sudo et al, Technical Digest, Fourth International Conference on Integrated Optics and Optical Fiber Communication, Tokyo (1983), 27A3-4. The technique of the '431 patent comprises forming a glass body of fusing alkoxide-derived glass particles to a glass substrate by means of an oxygen plasma torch.
Among the advantages of the alkoxide process are its relatively low processing temperatures and its potential for economical production of high purity glass. The conventional alkoxide process also has several shortcomings. For instance, it is frequently difficult to produce chemically homogeneous doped or mixed oxide glasses by means of the alkoxide process. In such multi-component glass systems, homogeneity generally can be achieved only if the hydrolization rates of all precursor materials are substantially the same. This, however, is rarely the case. In order to deal with this problem, methods for prehydrolyzing slowly hydrolyzing species have been proposed. Use of such methods complicates the overall process; and, despite their use, controlled gelation is generally difficult to achieve in multi-component systems. A further technique for dealing with different hydrolization rates of precursor materials is disclosed in U.S. Pat. No. 4,477,580.
Among other shortcomings of the alkoxide process is the requirement that all precursor materials be in solution. However, some important precursors (e.g., TEOS) are substantially insoluble in water. Therefore, it is frequently required to add an otherwise unnecessary constituent (e.g., ethyl alcohol) to the system, to facilitate dissolution. Furthermore, alkoxides typically are synthesized from simpler metal halides, e.g., SiCl.sub.4. Such processing obviously results in a product that is more costly, and frequently is less pure, than the starting metal halide. The alkoxide process also typically is relatively difficult to control reliably, since a host of parameters (e.g., pH, temperature, pressure, volume, composition, catalyst, degree of agitation, and sequence and rate of combination) all affect the outcome. Another disadvantage is the often excessively long gelation time, which can be, in some circumstances, days to weeks, and rarely is less than several hours.
Among the articles that can potentially comprise a sol/gel-derived glass body is optical fiber drawn from a fiber preform. Such fiber typically is silica-based and comprises a core contactingly surrounded by a cladding, with the former having a higher refractive index than the latter to achieve guiding of electromagnetic radiation of an appropriate wavelength, e.g., in the range of 0.7-1.6 .mu.m. The refractive index difference is produced, for instance, by incorporating an up-dopant (a dopant which increases the refractive index, e.g., GeO.sub.2) into the core region and/or incorporating a down-dopant (a dopant which decreases the refractive index, e.g., fluorine) into the cladding.
Other examples of articles that can potentially comprise a sol/gel-derived glass body are lenses and prisms, and high silicas glass tubes such as are used as substrate tubes in the MCVD process and in many other industrial processes, e.g., in semiconductor processing.
In view of the potential advantages of producing glass by a sol/gel process, a method for forming a monolithic gel that is free of many of the shortcomings of the prior art polymerization processes, especially of the alkoxide process, would be of great significance. This application discloses such a method.